This invention relates to a whole body counter for detecting and counting internal and external radiation contamination of a human subject.
Strict regulation of the nuclear power plants in operation has led to requirements that power plants carefully monitor their personnel to detect any internal radiation contamination, and to determine the extent of any contamination which is detected. During normal operation of a nuclear power plant, radiation detectors are routinely used to detect contamination of plant personnel. Government regulations require that a dosimetry record be reported to the Nuclear Regulatory Commission for each individual working in a plant. In particular, it is required that a worker's radiation dose be determined both at the time the worker is assigned to a plant and at the time the worker leaves the plant for a new assignment. This is done both to ensure the safety of the workers in nuclear power plants, and to develop a contamination profile to determine which nuclear power plants are producing the greatest amount of contamination.
During normal operation of a nuclear power plant, the preparation of the dosimetry records does not present any particular difficulties since there are normally 200 to 300 people working at a plant. However, nuclear power plants are in normal operation for approximately 18 months, after which time the plant must be shut down for approximately 6 weeks for refueling and maintenance. During this maintenance period, as many as 2,000 to 3,000 workers will work in the plant. Since these workers are assigned to the plant and leave the plant within this six week period, it becomes a cumbersome task to develop the necessary dosimetry records during this time period. This is further compounded by the fact that many of the workers will need to enter the plant (and thus have their dose determined before entry) at the beginning of the six week maintenance period. Because of the large volume of monitoring which must take place during the maintenance period, many companies temporarily obtain auxiliary whole body counters housed in portable trailers for use during the maintenance period to perform the necessary radiation monitoring. These auxiliary whole body counters are used in addition to the whole body counters which are used at the lant during normal operation, in order to handle the large volume of personnel which must be monitored during the maintenance period.
Early whole body counters were placed in small shielded rooms with a bed. Later, the so-called shadow shield design was developed which was much smaller in size. Currently, this shadow shield whole body counter design is in wide use. However, currently available whole body counters are relatively slow in that they require 8 to 10 minutes to perform a detection procedure for a single subject. In addition, currently available whole body counters have low resolution because they employ sodium iodide detectors (approximately 8% resolution). The resolution refers to the ability of a detector system to resolve a peak at a specific energy, and is best shown by a graph of the number of events of energy versus the energy level, where the particular energy level is used to identify the type of radionuclide (e.g., cesium 137, cobalt 60, etc.). While sodium iodide detectors are relatively inexpensive, their low resolution impairs the accuracy of the information produced when several different energies of radiation contamination are detected. A sodium iodide detector will produce a characteristic (number of events of energy versus energy level) having rounded peaks which are spread out, thereby obscuring information for identifying the particular radionuclide. Since the amount of radiation is determined by integrating the area under a peak, the more a peak is spread out, the more inaccurate the radiation reading will be. Thus, sodium iodide detectors have relatively low resolution. A direct result of this low resolution is that sodium iodide detectors require 2 to 3 minutes of computer time to process the output signal of the detector because of the complex program required for filtering noise from the output signal.
Another disadvantage of currently available whole body counters is that they are not capable of separately identifying whether the contamination is internal or external. Internal and external radiation monitoring differ because of the penetrating ability of the different types of radiation. Beta radiation has a low penetrating ability and therefore can only be monitored on the exterior of the subject, while gamma radiation has a relatively high penetrating ability, so that it will tend to penetrate through at least a portion of the subject. For example, gamma radiation is used for X-ray purposes. The external contamination will include gamma radiation and beta radiation. This phenomenon will interfere with the gamma rays emitted from inside the subject's body. Therefore, it is important to evaluate the external contamination to prevent the false reporting of internal contamination. Nuclear power plants will typically be subject to the presence of both beta and gamma radiation.
While most nuclear power plants preliminarily employ a hand-held scanner to check for external radiation on a subject, such scans are not always accurate. When currently available whole body counters using sodium iodide detectors are employed, the radiation which is detected can be external or internal. Further, with available whole body counters, the information developed by the counter is typically evaluated at a remote location, so that the contamination which is detected by the sodium iodide detector may not be immediately identified as external contamination. In fact, the presence of external contamination may not be determined for some time. The existance of a delay in determining that contamination is external is disadvantageous. Thus, a significant time delay occurs in the time for determining the nature of any internal contamination. In addition, the lengthy original scan provides little useful information because of the occurrence of the external contamination.
There is a need in the art for a radiation detector which is capable of quickly detecting the presence of any external radiation, and which is capable of providing accurate information concerning the presence of any internal contamination.